1. Field of the Invention
The present invention is related generally to wireless communication devices, and, more particularly, to a system and method for distortion reduction in a wireless communication device.
2. Description of the Related Art
The present invention provides this, and other advantages, as will be apparent from the following detailed description and the accompanying figures.
Wireless communication systems are proliferating as more and more service providers add additional features and technical capabilities. A large number of service providers now occupy a relatively limited portion of the radio frequency spectrum. Due to this crowding, increased interference between wireless communication systems is commonplace. For example, wireless communication systems from two different service providers may occupy adjacent portions of the spectrum. In this situation, interference may be likely.
One example of such interference occurs in a code division multiple access (CDMA) wireless system. In one embodiment, a CDMA system occupies a portion of the frequency spectrum adjacent to a portion of the frequency spectrum allocated to a conventional cellular telephone system, sometimes referred to as an advanced mobile phone system (AMPS).
Conventional CDMA units attempt to eliminate undesirable signals by adding filters following the mixer stage. FIG. 1 illustrates one known implementation of a direct-to-baseband or low IF wireless system 10 in which a radio frequency (RF) stage 12 is coupled to an antenna 14. The output of the RF stage 12 is coupled to an amplifier 16, which amplifies the radio frequency signals. It should be noted that the RF stage 12 and the amplifier 16 may include conventional components such as amplifiers, filters, and the like. The operation of these stages is well known and need not be described in greater detail herein.
The output of the amplifier 16 is coupled to a splitter 18 that splits the processed signal into two identical signals for additional processing by a mixer 20. The splitter 18 may be an electronic circuit or, in its simplest form, just a wire connection. The mixer 20 comprises first and second mixer cores 22 and 24, respectively. The mixers 22 and 24 are identical in nature, but receive different local oscillator signals. The mixer core 22 receives a local oscillator signal, designated LOI, while the mixer core 24 receives a local oscillator signal, designated as LOQ. The local oscillator signals are 90° out of phase with respect to each other, thus forming a quadrature mixer core. The output of the mixer 20 is coupled to jammer rejection filter stage 26. Specifically, the output of the mixer core 22 is coupled to a jammer rejection filter 28 while the output of the mixer core 24 is coupled to a jammer rejection filter 30. The operation of the jammer rejection filters 28 and 30 is identical except for the quadrature phase relationship of signals from the mixer 20. The output of the jammer rejection filters 28 and 30 are the quadrature output signals IOUT and QOUT respectively.
The jammer rejection filters 28 and 38 are designed to remove unwanted signals, such as signals from transmitters operating at frequencies near the frequency of operation of the system 10. Thus, the jammer rejection filters 28 and 30 are designed to remove “out-of-band” signals. In operation, the jammer rejection filters 28 and 30 may be lowpass filters, bandpass filters, or complex filters (e.g., a single filter with two inputs and two outputs), depending on the implementation of the system 10. The operation of the jammer rejection filters 28 and 30 are well known in the art and need not be described in greater detail herein. While the jammer rejection filters 28 and 30 may minimize the effects of out-of-band signals, there are other forms of interference for which the jammer rejection filters are ineffective.
For example, distortion products created by the mixer 20 may result in interference that may not be removed by the jammer rejection filters 28 and 30. If one considers a single CDMA wireless unit, that unit is assigned a specific radio frequency or channel in the frequency spectrum. If an AMPS system is operating on multiple channels spaced apart from each other by a frequency ΔωJ, then the second-order distortion from the mixer 20 will create a component at a frequency ΔωJ in the output signal. It should be noted that the second order distortion from the mixer 20 will create signal components at the sum and difference of the two jammer frequencies. However, the signal resulting from the sum of the jammer frequencies is well beyond the operational frequency of the wireless device and thus does not cause interference. However, the difference signal, designated herein as ΔωJ, may well be inside the desired channel and thus cause significant interference with the desired signal.
In this circumstance, the AMPS signals are considered a jammer signals since they create interference and therefore jam the desirable CDMA signal. Although the present example refers to AMPS signals as jammer signals, those skilled in the art will appreciate that any other radio frequency sources spaced at a frequency of ΔωJ from each other may be considered a jammer.
If this second-order distortion signal is inside the channel bandwidth, the jammer rejection filters 28 and 30 will be ineffective and the resultant interference may cause an unacceptable loss of carrier-to-noise ratio. It should be noted that this interference may occur regardless of the absolute frequencies of the jammer signals. Only the frequency separation is important if the second-order distortion results in the introduction of an undesirable signal into the channel bandwidth of the CDMA unit.
Industry standards exist that specify the level of higher order distortion that is permitted in wireless communication systems. A common measurement technique used to measure linearity is referred to as an input-referenced intercept point (IIP). The second order distortion, referred to as IIP2, indicates the intercept point at which the output power in the second order signal intercepts the first order signal. As is known in the art, the first order or primary response may be plotted on a graph as the power out (POUT) versus power in (PIN) In a linear system, the first order response is linear. That is, the first order power response has a 1:1 slope in a log-log plot. The power of a second order distortion product follows a 2:1 slope on a log-log plot. It follows that the extrapolation of the second order curve will intersect the extrapolation of the fundamental or linear plot. That point of intercept is referred to as the IIP2. It is desirable that the IIP2 number be as large as possible. Specifications and industry standards for IIP2 values may vary from one wireless communication system to another and may change over time. The specific value for IIP2 need not be discussed herein.
It should be noted that the second-order distortion discussed herein is a more serious problem using the direct down-conversion architecture illustrated in FIG. 1. In a conventional super-heterodyne receiver, the RF stage 12 is coupled to an intermediate frequency (IF) stage (not shown). The IF stage includes bandpass filters that readily remove the low frequency distortion products. Thus, second-order distortion is not a serious problem with a super-heterodyne receiver. Therefore, the IIP2 specification for a super-heterodyne receiver is generally not difficult to achieve. However, with the direct down-conversion receiver, such as illustrated in FIG. 1, any filtering must be done at the baseband frequency. Since the second-order distortion products at the frequency separation, ΔωJ, regardless of the absolute frequency of the jammers, the IIP2 requirements are typically very high for a direct-conversion receiver architecture. The IIP2 requirement is often the single most difficult parameter to achieve in a direct down-conversion receiver architecture.
As noted above, the second-order distortion is often a result of non-linearities in the mixer 20. There are a number of factors that lead to imbalances in the mixer 20, such as device mismatches (e.g., mismatches in the mixer cores 22 and 24), impedance of the local oscillators, and impedance mismatch. In addition, factors such as the duty cycle of the local oscillator also has a strong influence on the second-order distortion. While various calibration schemes may be effective to reestablish symmetry in the mixer 20, this calibration is time-consuming and will vary from one wireless unit to the next due to component variations. Such time-consuming calibration processes are expensive do not lend themselves well to high-volume production since the calibration must be done initially at the factory and may require periodic recalibration during normal use.
In addition to the calibration difficulties, the undesirable second-order products are generally quite low relative to the desired signal and are thus not directly measurable. All these factors lead to difficulty in reducing the second-order distortion products to an acceptable level. Therefore, it can be appreciated that there is a significant need for a system and method for wireless communication that reduces the undesirable distortion products to an acceptable level. The present invention provides this and other advantages as will be apparent from the following detailed description and accompanying figures.